Railway tank cars may be used for carrying a variety of liquid, gaseous, and semi-liquid ladings. Such railway tank cars often include a horizontal, generally cylindrical tank. For center stub sill or draft sill type tank cars, the tank not only carries the lading, but is also used as a primary .structural member by the railway car to carry longitudinal train loads. These cars typically have center stub sills or draft sills on each end of the car along with transverse bolster assemblies. The draft sills carry couplers at their outer ends and have truck assemblies pivotally connected thereto for rolling support of the ends of the tank car on the railway tracks. Saddle assemblies, or cradle assemblies, are typically provided at each end of the car. The saddle assembly usually comprises a circumferential saddle extending arcuately and transversely up from the bottom of the car and mating with a saddle pad, or bolster pad, that is attached to the tank. The saddle and saddle pad are typically of steel plate construction and conformed to fit onto the adjacent outer surface of the cylindrical tank. Each end of the tank typically also has a head pad, or front sill pad.
The pads, such as the front sill pad and saddle pad, serve to distribute the loads and stresses over a larger area and to help prevent rupture of the associated tank. The connection between the pads and the tank is designed to be stronger than between the pads and other components welded to the pad. This arrangement enhances safety by making rupture of the tank less likely.
In securing the front sill pad to the draft sill, fillet and groove welds on the exterior of the head brace have conventionally been used. The head brace is typically a U-shaped, wedge-like piece that is placed between the front sill pad and portions of the draft sill to facilitate attachment of the tank to the draft sill, and to reduce the geometric stress concentrations at the outboard attachment points. As noted above, it has typically been conventional to apply the head brace with welding only on the exterior of the head brace. This attachment has been sufficient to meet the strength criteria required by the Association of American Railroads (AAR).
In recent times, the tank car industry has discovered a significant number of fatigue cracks on the head braces or the connections associated with the head braces. The frequency of fatigue cracks in this area may be as high as fifty percent of tank cars that have been on the railway tracks for a sufficient time period.